1. Field of the Invention
The measurement of trace amounts of a wide variety of organic compounds has become essential in medicine, ecology, quality control, and the like. One class of methods commonly referred to as immunoassays is dependent upon the use of a compound or receptor which specifically binds to another compound having a particular spatial and polar organization. The compound and its receptor form a homologous pair, referred to as ligand and receptor, where the receptor is normally antibody. One of the members of the homologous pair is bound to a label which is capable of providing a detectible signal.
The category of immunoassays may be further broken down into what is referred to as heterogeneous and homogeneous. The heterogeneous techniques are dependent upon separating associations or complexes of the homologous pair from members of the pair which are not associated. Since the complexes will substantially differ in molecular weight from the dissociated members, techniques such as centrifugation can be used to separate the associated from the dissociated members. One can then measure the label either in the phase containing the dissociated members or the phase containing the associated members. For the most part the labels which have found use in the heterogeneous methods are radiolabels, enzymes, and fluorescent molecules.
An alternative to physical separation is to bind one of the members of the homologous pair to a solid support, which may or may not absorb the aqueous medium. The solid support can then provide for the separation since the complexed or associated ligand and receptor is bound to the solid support. This allows for relatively easy separation between the aqueous assay medium and the solid support.
The homogeneous methods rely on the formation of complexes to modulate the signal obtained from the label. The dissociated conjugated label provides for a different level of signal from the conjugated label associated with its receptor. For example, where the ligand is conjugated to a stable free radical, the association of the conjugate with its homologous receptor results in a substantial flattening of the esr peaks. With enzymes as labels to which ligands have been conjugated, the binding of receptor to the ligands can result in steric inhibition of the approach of substrate to the active site of the enzyme or allosteric modification of enzyme activity. The presence of ligand in the assay medium reduces the amount of available receptor for binding to the label conjugate and thus affects the amount of the label conjugate which becomes associated with receptor. Therefore, by measurement of the signal from the label, one can relate the level of signal to the amount of ligand in the assay medium.
An alternative to employing the receptor to directly affect the signal by its bulk is the opportunity to bring together two labels which interact. Where a ligand is polyepitopic or a polyepitopic ligand is formed from monoepitopic ligands, the opportunity exists to allow for receptors which are labeled differently to be brought together when bound to the ligand or to have ligand with one label and receptor with a different label, which when the ligand and receptor are associated bring the labels into close spatial proximity. Where the different labels interact to affect the amount of signal observed, the associated ligand and receptor will provide for a different signal level from the dissociated labeled receptor.
This technique has been employed with chromophores which are related by one of the chromophores fluorescing at a wavelength of an energy which is accepted by the other chromophore, which acts as a quencher. Also, by employing two different enzymes, where the product of one enzyme is the substrate of the other enzyme, one can observe an enhanced turnover in the complex, as compared to the dissociated label.
The focus of effort in the homogeneous immunoassay area has been directed to either employ the properties of the complex to modulate the signal or to provide for the complex to bring together in close spatial proximity different labels which are related and provide for different degrees of interaction in relation to their distance from each other.
In developing immunoassays, there are many considerations, not the least of which is sensitivity. For measuring extremely small amounts of a ligand, it is either necessary to have a label which is detected at very low levels with high accuracy or to provide for a plurality of events associated with an individual ligand. Another consideration is interference by the foreign materials present and the degree to which the interference can be minimized or removed.
Another problem associated with immunoassays is labeling, particularly where the ligand or receptor is impure. The background resulting from conjugation of the label to compounds other than those of the homologous pair must be maintained at a minimum in order to obtain a satisfactorily sensitive assay. Other considerations include simplicity of protocol, ease of measurement, reproducibility, sensitivity to extraneous factors and the like.